In printing presses, the paper web being printed is subjected to the application of water and/or ink that absorbs into the paper and produces a size change in the paper. This change in size occurs both longitudinally (machine direction (MD)) and width wise (cross machine direction CD)) of the web. Dimensional changes caused in each printing station must be accommodated in the next printing station to ensure print registration, i.e. when multicolored printing is being performed, expansion may cause some misregister of subsequently applied colors. This misregistration in the machine direction is currently compensated for by adjusting the web tension and in effect, jogging selected printing station(s) when significant longitudinal misregister is detected. Lateral or width wise misregister due to CD dimension changes as the web passes through the press is more difficult to accommodate, as prior to the present invention, the concept of measuring and how to measure the CD dimensional change was not available. Techniques for adjusting for CD expansion were based solely on examination of the printed copy and generally, subjectively judging the misregistration.
Correction of lateral (width wise) misregistration is a matter of trial and error and is difficult to compensate for as subjective assessment must be translated into specific action.
Also, every time the pattern to be printed is changed or the paper to be printed on is changed, the amount of liquid (water) absorbed is changed and the amount of expansion changes.
It is known to detect an edge of a traveling web using a laser beam by monitoring the portion of the laser beam either beyond the edge of the web or blocked by the edge of the web.
It is also known to sense the thickness of elements and/or the width of narrow tapes or the like of less width than the length of a laser line used in the sensor. Sensing the location of opposite ends of a board using reflecting laser beams and then determining the width of the board is also known.